Switch mode power supplies or switching regulators, also referred to as DC-to-DC converters, are often used to convert an input supply voltage to a desired output voltage at a voltage level appropriate for the internal circuitry of an integrated circuit. For example, a 5 volts supply voltage provided to an integrated circuit may need to be reduced to 2.8 volts on the IC chip to operate the internal circuitry on the chip. A switching regulator provides power supply function through low loss components such as capacitors, inductors, and transformers, and power switches that are turned on and off to transfer energy from the input to the output on cycle by cycle basis. A feedback control circuit is used to regulate the energy transfer to maintain a constant output voltage within the desired load limits of the circuit.
The operation of the conventional switching regulator is well known and is generalized as follows. A conventional step down (buck) switching regulator includes a pair of power switches which are turned on and off to regulate an output voltage to be equal to a reference voltage. More specifically, the power switches are alternately turned on and off to generate a switching output voltage at a switching output node, also referred to as the switch node. The switch node is coupled to an LC filter circuit including an output inductor and an output capacitor to generate an output voltage having a substantially constant magnitude. The output voltage can then be used to drive a load.
More specifically, the pair of power switches is often referred to as a “high-side power switch” and a “low-side power switch.” The high-side power switch is turned on to apply energy to the output inductor of the output filter circuit to allow the current through the inductor to build up. When the high-side power switch is turned off, the voltage across the inductor reverses and the current through the inductor reduces. This insures that the inductor current ripples above and below the nominal output current. A relatively constant output voltage is maintained by the output capacitor. The low-side power switch is turned on and off for synchronous control operation.
FIG. 1 is a schematic diagram of a conventional switching regulator. Referring to FIG. 1, a switching regulator 1 includes a switching regulator controller 10 coupled to drive a pair of power switches S1 and S2 connected in series between the input voltage VIN and a ground potential. Power switches S1 and S2 are alternately turned on and off to generate a switching output voltage VSW at a switch node (SW) 22. The switching output voltage VSW is directly coupled to an LC filter circuit including an output inductor L1 and an output capacitor COUT to generate a regulated output voltage VOUT at a node 26 having a substantially constant magnitude. The output voltage VOUT can then be used to drive a load 30 whereby switching regulator 1 provides the load current ILOAD to maintain the output voltage VOUT at a constant level.
Switching regulator 1 includes a feedback control circuit to regulate the energy transfer to the LC filter circuit to maintain a constant output voltage within the desired load limits of the circuit. More specifically, the feedback control circuit causes power switches S1 and S2 to turn on and off to regulate the output voltage VOUT to be equal to a reference voltage VREF or to a voltage value related to the reference voltage VREF. In the present embodiment, a voltage divider including resistors R1 and R2 is used to divide down the output voltage VOUT which is then fed back to the switching regulator controller 10 as a feedback voltage VFB on a feedback node 28. The feedback voltage VFB is compared with the reference voltage VREF at an error amplifier 12. The difference between the feedback voltage VFB and the reference voltage VREF is coupled to a control circuit 14 to generate control voltages for the power switches based on a switching regulator control scheme. The control voltages are then provided to a gate drive circuit 16 to generate gate drive signals for the power switches S1 and S2. The gate drive signal for the high-side power switch S1 is coupled to a high-side driver circuit 18 while the gate drive signal for the low-side power switch S2 is coupled to a low-side driver circuit 20. Driver circuits 18, 20 convert the respective gate drive signals to gate drive voltages appropriate for turning on and off the respective power switches. The gate drive signals for the high-side power switch S1 and the low-side power switch S2 are opposite in polarities so that one power switch is turned on when the other one is turned off.
Some switching regulators employ pulse width modulation (PWM) to control the duty cycle of the power switches S1 and S2. That is, the control circuit 14 controls the ON-time of high-side power switch S1 at a fixed frequency by adjusting the pulse width. Conventional switching regulators employing PWM control are often configured using a PWM controller driving the power switches, as shown in FIG. 2. As shown in FIG. 2, a PWM controller 40 includes a PWM control circuit 42 for generating the PWM clock signal for the gate drive circuit 16 to control the power switches S1 and S2. The switching regulator of FIG. 2 can be configured using various levels of integration. For example, in some cases, the PWM controller 40 is formed as an integrated circuit where the power switches, the output inductor and the output capacitor are formed as discrete off-chip components. In other cases, the power switches and/or the output inductor/capacitor may be integrated onto the same integrated circuit as the PWM controller.
Furthermore, a switching regulator employing PWM control may further implement different control schemes, including voltage mode control or current mode control (also referred to as peak current mode control or valley current mode control). However, conventional PWM controllers for switching regulators are designed to operate using one specific control scheme. A system designer using a PWM controller designed with one control scheme wishing to change to a PWM controller with another control scheme to improve the performance of the switching regulator would have to change the controller integrated circuit. Changing the controller integrated circuit often involves changing the PC board design which leads to increase cost and causes delay in the design cycle.